A. Tomal

Evaluation of scatter-to-primary ratio, grid performance and normalized average glandular dose in mammography by Monte Carlo simulation including interference and energy broadening effects

In this work, a computational code for the study of imaging systems and dosimetry in conventional and digital mammography through Monte Carlo simulations is described. The developed code includes interference and Doppler energy broadening for simulation of elastic and inelastic photon scattering, respectively. The code estimates the contribution of scattered radiation to image quality through the spatial distribution of the scatter-to-primary ratio (S/P). It allows the inclusion of different designs of anti-scatter grids (linear or cellular), for evaluation of contrast improvement factor (CIF), Bucky factor (BF) and signal difference-to-noise ratio improvement factor (SIF). It also allows the computation of the normalized average glandular dose, D(g).(N). These quantities were studied for different breast thicknesses and compositions, anode/filter combinations and tube potentials. Results showed that the S/P increases linearly with breast thickness, varying slightly with breast composition or the spectrum used. Evaluation of grid performance showed that the cellular grid provides the highest CIF with smaller BF. The SIF was also greater for the cellular grid, although both grids showed SIF < 1 for thin breasts. Results for D(g).(N) showed that it increases with the half-value layer (HVL) of the spectrum, decreases considerably with breast thickness and has a small dependence on the anode/filter combination. Inclusion of interference effects of breast tissues affected the values of S/P obtained with the grid up to 25%, while the energy broadening effect produced smaller variations on the evaluated quantities.

Response functions of Si(Li), SDD and CdTe detectors for mammographic x-ray spectroscopy.

In this work, the energy response functions of Si(Li), SDD and CdTe detectors were studied in the mammographic energy range through Monte Carlo simulation. The code was modified to take into account carrier transport effects and the finite detector energy resolution. The results obtained show that all detectors exhibit good energy response at low energies. The most important corrections for each detector were discussed, and the corrected mammographic x-ray spectra obtained with each one were compared. Results showed that all detectors provided similar corrected spectra, and, therefore, they could be used to accurate mammographic x-ray spectroscopy. Nevertheless, the SDD is particularly suitable for clinic mammographic x-ray spectroscopy due to the easier correction procedure and portability.

Monte Carlo simulation of the response functions of CdTe detectors to be applied in x-ray spectroscopy.

In this work, the energy response functions of a CdTe detector were obtained by Monte Carlo (MC) simulation in the energy range from 5 to 160keV, using the PENELOPE code. In the response calculations the carrier transport features and the detector resolution were included. The computed energy response function was validated through comparison with experimental results obtained with (241)Am and (152)Eu sources. In order to investigate the influence of the correction by the detector response at diagnostic energy range, x-ray spectra were measured using a CdTe detector (model XR-100T, Amptek), and then corrected by the energy response of the detector using the stripping procedure. Results showed that the CdTe exhibits good energy response at low energies (below 40keV), showing only small distortions on the measured spectra. For energies below about 80keV, the contribution of the escape of Cd- and Te-K x-rays produce significant distortions on the measured x-ray spectra. For higher energies, the most important correction is the detector efficiency and the carrier trapping effects. The results showed that, after correction by the energy response, the measured spectra are in good agreement with those provided by a theoretical model of the literature. Finally, our results showed that the detailed knowledge of the response function and a proper correction procedure are fundamental for achieving more accurate spectra from which quality parameters (i.e., half-value layer and homogeneity coefficient) can be determined.

Optimization of x-ray spectra in digital mammography through Monte Carlo simulations

In this work, a Monte Carlo code was used to investigate the performance of different x-ray spectra in digital mammography, through a figure of merit (FOM), defined as FOM = CNR²/(¯)D(g), with CNR being the contrast-to-noise ratio in image and [Formula: see text] being the average glandular dose. The FOM was studied for breasts with different thicknesses t (2 cm ≤ t ≤ 8 cm) and glandular contents (25%, 50% and 75% glandularity). The anode/filter combinations evaluated were those traditionally employed in mammography (Mo/Mo, Mo/Rh, Rh/Rh), and a W anode combined with Al or K-edge filters (Zr, Mo, Rh, Pd, Ag, Cd, Sn), for tube potentials between 22 and 34 kVp. Results show that the W anode combined with K-edge filters provides higher values of FOM for all breast thicknesses investigated. Nevertheless, the most suitable filter and tube potential depend on the breast thickness, and for t ≥ 6 cm, they also depend on breast glandularity. Particularly for thick and dense breasts, a W anode combined with K-edge filters can greatly improve the digital technique, with the values of FOM up to 200% greater than that obtained with the anode/filter combinations and tube potentials traditionally employed in mammography. For breasts with t < 4 cm, a general good performance was obtained with the W anode combined with 60 μm of the Mo filter at 24-25 kVp, while 60 μm of the Pd filter provided a general good performance at 24-26 kVp for t = 4 cm, and at 28-30 and 29-31 kVp for t = 6 and 8 cm, respectively.

Characterization of Tissue-Equivalent Materials Through Measurements of the Linear Attenuation Coefficient and Scattering Profiles Obtained With Polyenergetic Beams

In this work seven tissue-equivalent materials (Nylon, Polyacetate, Polymethylmethacrylate (PMMA), water, muscle-equivalent, bone-equivalent and adipose-equivalent) were characterized, through their attenuation (linear attenuation coefficient) and scattering (scattering profile) properties. An energy dispersive X-ray system (EDXS) was used to analyze these properties simultaneously. The EDXS consisted of a tungsten anode X-ray tube operating at 60 kVp, a goniometer, and two detectors: a Cadmium Telluride (CdTe) detector, positioned at 7 degrees with relation to the incident beam, used for detecting the energy distribution of the scattered photons, and a Silicon Drift Detector (SDD), positioned at zero degree with relation to the incident beam, used for detecting the energy distribution of the transmitted beam (with the sample) or the incident beam (without the sample). The preliminary results obtained in this work show the potential of combining the linear attenuation coefficient and the scattering profile for characterizing and choosing the most suitable tissue-equivalent materials to simulate human tissue. Our results show that adipose-equivalent, water and bone-equivalent would be adequate to simulate adipose, muscle and bone tissue respectively.

Optimal X-Ray Spectra Selection in Digital Mammography: A Semi-Analytical Study

In this work, a semi-analytical model was developed to determine the contrast-to-noise ratio (CNR) and the normalized average glandular dose (D̅gN) in digital mammography, taking into account interactions up to second order. The optimal x-ray spectra were investigated using a Figure of Merit (FOM), for different breast thicknesses (3 cm ≤ L ≤ 7 cm) and glandularities (20% and 50%), anti-scatter grids types, considering a-Se and CsI image receptors. The anode/filter combinations evaluated were: Mo/Mo, Mo/Rh, Rh/Rh, and a W anode combined with k-edge filters (Mo, Ru, Rh, Pd, Ag, Cd, In and Sn), for tube potential between 23 and 35 kVp. Results demonstrate that the W-anode spectra combined with k-edge filters show a superior performance compared to the Mo/Mo combination traditionally employed in mammography, for all conditions analyzed. The optimum general x-ray spectra for 3 cm, 5 cm and 7 cm breasts are, respectively, W/Mo at 23 kVp, W/Pd at 28 kVp and W/Ag at 30 kVp. For the a-Se receptor, the optimum filter atomic number is one unit higher than that obtained for the CsI, while the optimal tube potentials are the same. The optimal tube potential was up to 1-2 kVp greater for a linear grid than that obtained without grid or with a cellular grid. For breast thicker than 7 cm, the optimum filter atomic number also increases by one unit for a linear grid. Compared to standard technique, the performance improvement of a W anode with proper k-edge filters is more noticeable for thicker and denser breasts, for a a-Se receptor and a linear grid. Finally, it was verified that the semi-analytical model implemented provided results of optimum x-ray spectra in a fast and simple way, with a good agreement with those obtained experimentally or by Monte Carlo simulation.

Evaluation of conversion coefficients relating air-kerma to H*(10) using primary and transmitted x-ray spectra in the diagnostic radiology energy range.

According to the International Commission on Radiation Units and Measurements (ICRU), the relationship between effective dose and incident air-kerma is complex and depends on the attenuation of x-rays in the body. Therefore, it is not practical to use this quantity for shielding design purposes. This correlation is adopted in practical situations by using conversion coefficients calculated using validated mathematical models by the ICRU. The ambient dose equivalent, H*(10), is a quantity adopted by the IAEA for monitoring external exposure. Dose constraint levels are established in terms of H*(10), while the radiation levels in radiometric surveys are calculated by means of the measurements of air-kerma with ion chambers. The resulting measurements are converted into ambient dose equivalents by conversion factors. In the present work, an experimental study of the relationship between the air-kerma and the operational quantity ambient dose equivalent was conducted using different experimental scenarios. This study was done by measuring the primary x-ray spectra and x-ray spectra transmitted through materials used in dedicated chest radiographic facilities, using a CdTe detector. The air-kerma to ambient dose equivalent conversion coefficients were calculated from these measured spectra. The resulting values of the quantity ambient dose equivalent using these conversion coefficients are more realistic than those available in the literature, because they consider the real energy distribution of primary and transmitted x-ray beams. The maximum difference between the obtained conversion coefficients and the constant value recommended in national and international radiation protection standards is 53.4%. The conclusion based on these results is that a constant coefficient may not be adequate for deriving the ambient dose equivalent.

Application of a semi-empirical model for the evaluation of transmission properties of barite mortar

The aim of this study was to estimate barite mortar attenuation curves using X-ray spectra weighted by a workload distribution. A semi-empirical model was used for the evaluation of transmission properties of this material. Since ambient dose equivalent, H(⁎)(10), is the radiation quantity adopted by IAEA for dose assessment, the variation of the H(⁎)(10) as a function of barite mortar thickness was calculated using primary experimental spectra. A CdTe detector was used for the measurement of these spectra. The resulting spectra were adopted for estimating the optimized thickness of protective barrier needed for shielding an area in an X-ray imaging facility.

Monte Carlo Simulation of X-Ray Spectra in Mammography and Contrast-Enhanced Digital Mammography Using the Code PENELOPE

In this work, the Monte Carlo (MC) code PENELOPE was employed for simulation of x-ray spectra in mammography and contrast-enhanced digital mammography (CEDM). Spectra for Mo, Rh and W anodes were obtained for tube potentials between 24-36 kV, for mammography, and between 45-49 kV, for CEDM. The spectra obtained from the simulations were analytically filtered to correspond to the anode/filter combinations usually employed in each technique (Mo/Mo, Rh/Rh and W/Rh for mammography and Mo/Cu, Rh/Cu and W/Cu for CEDM). For the Mo/Mo combination, the simulated spectra were compared with those obtained experimentally, and for spectra for the W anode, with experimental data from the literature, through comparison of distribution shape, average energies, half-value layers (HVL) and transmission curves. For all combinations evaluated, the simulated spectra were also compared with those provided by different models from the literature. Results showed that the code PENELOPE provides mammographic x-ray spectra in good agreement with those experimentally measured and those from the literature. The differences in the values of HVL ranged between 2-7%, for anode/filter combinations and tube potentials employed in mammography, and they were less than 5% for those employed in CEDM. The transmission curves for the spectra obtained also showed good agreement compared to those computed from reference spectra, with average relative differences less than 12% for mammography and CEDM. These results show that the code PENELOPE can be a useful tool to generate x-ray spectra for studies in mammography and CEDM, and also for evaluation of new x-ray tube designs and new anode materials.

Direct measurement of clinical mammographic x‐ray spectra using a CdTe spectrometer

Purpose To introduce and evaluate a method developed for the direct measurement of mammographic x‐ray spectra using a CdTe spectrometer. The assembly of a positioning system and the design of a simple and customized alignment device for this application is described. Methods A positioning system was developed to easily and accurately locate the CdTe detector in the x‐ray beam. Additionally, an alignment device to line up the detector with the central axis of the radiation beam was designed. Direct x‐ray spectra measurements were performed in two different clinical mammography units and the measured x‐ray spectra were compared with computer‐generated spectra. In addition, the spectrometer misalignment effect was evaluated by comparing the measured spectra when this device is aligned relatively to when it is misaligned. Results The positioning and alignment of the spectrometer have allowed the measurements of direct mammographic x‐ray spectra in agreement with computer‐generated spectra. The most accurate x‐ray spectral shape, related with the minimal HVL value, and high photon fluence for measured spectra was found with the spectrometer aligned according to the proposed method. The HVL values derived from both simulated and measured x‐ray spectra differ at most 1.3 and 4.5% for two mammography devices evaluated in this study. Conclusion The experimental method developed in this work allows simple positioning and alignment of a spectrometer for x‐ray spectra measurements given the geometrical constraints and maintenance of the original configurations of mammography machines.